The small print on the page that mentions this stat says that the measurements were taken on a new 12.9 inch iPad Pro using the Gen 2 Apple Pencil. Not sure what latency reductions may make their way to the other iPads, but nothing is stated right now.

I have the 10.5 in Pro and Apple Pencil 1. I wonder if the 20 ms vs 9 ms is detectable by the human eye.

It can be very detectable, I'd say. Try watching this: https://www.youtube.com/watch?v=vOvQCPLkPt4 It shows the difference between from >100ms to ~1ms. It's mind baffling.

I wonder how did they manage it. Even the Screen itself has a 1 - 4ms latency. That is excluding the Input latency and Software in between.

It might be. On monitors the difference between a 120hz refresh rate and 144hz refresh rate is about 14ms between frames. Some people (claim) to tell the difference.